MPRIS2 support (for indicator-sound and Gnome Shell media player extension)

YouTube plugin?

Rethink the gstreamer-plugins-installer (packagekit)

Include default theming for GtkOverlay in case themes don't support the osd class

doesn't seem possible, cause either the cssprovider overrides the theme or the theme overrides the fallback, because there doesn't seem to be a sane way to check whether the osd-class gets specially themed